The present invention relates to a method for determining a qualitative characteristic of an interferometric component (eg a planar waveguide structure) or a process for determining a qualitative characteristic of a stimulus of interest to which the interferometric component (eg the planar waveguide structure) has been exposed.
Generally speaking, it is known to make use of the evanescent field component of electromagnetic radiation incident on a planar waveguide structure (ie the field which extends outside the guiding region) to sense discrete changes in optical properties (see inter alia GB-A-2228082, U.S. Pat. No. 5,262,842, WO-A-97/12225 and GB-A-2307741). This technique relies on “leakage” of optical signals from a planar waveguide into a sensing layer typically formed from an absorbent polymer.
Based on the principles of interferometry, the use of a planar waveguide structure to quantify (ie to detect the amount of (eg concentration of) or changes in) a stimulus of interest is disclosed in inter alia WO-A-98/22807 and WO-A-01/36945. Such planar waveguide structures comprise a planar sensing waveguide or sensing layer and generate a plurality of interference fringes in an interference pattern. Positional measurements such as the movement of the interference fringes in response to the exposure of the planar waveguide structure to the stimulus of interest may be made to quantify the stimulus of interest. However such positional information is subject to limitations. The evanescent component of the optical signal being guided by the planar waveguide, is typically small (leading to limited interrogation of the sensing layer). Moreover the evanescent component varies exponentially and may extend beyond the sensing layer. This means that the aggregate response of the planar waveguide structure is non-linear being dependent on factors contributing to the change in the effective refractive index of the planar waveguide structure ie factors such as changes in dimension (eg physical thickness) of the sensing layer and/or changes in composition (ie intrinsic refractive index) of the sensing layer and beyond.
WO-A-01/36946 (Farfield Sensors Limited) discloses the use of TE and TM polarisations to differentiate factors (eg dimensional and compositional factors) contributing to changes in the effective refractive index of a planar waveguide structure. The response of the two polarisations are solved for consistency with a simple mono-adlayer model. However this method is limited by factors such as non-uniformity, optical activity or unknown composition of the sensing layer.